i915: Set object to gtt domain when faulting it back in

When a GEM object is evicted from the GTT we set it to the CPU domain,
as it might get swapped in and out or ever mmapped regularly.  If the
object is mmapped through the GTT it can still get evicted in this way
by other objects requiring GTT space.  When the GTT mapping is touched
again we fault it back into the GTT, but fail to set it back to the
GTT domain.  This means we fail to flush any cached CPU writes to the
pages backing the object which will then happen "eventually", typically
after we write to the page through the uncached GTT mapping.

[anholt: Note that userland does do a set_domain(GTT, GTT) when starting
to access the GTT mapping.  That covers getting the existing mapping of the
object synchronized if it's bound to the GTT.  But set_domain(GTT, GTT)
doesn't do anything if the object is currently unbound.  This fix covers the
transition to being bound for GTT mapping.]

Fixes glyph and other pixmap corruption during swapping.  fd.o bug #21790

Signed-off-by: Kristian Høgsberg <krh@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>